1. Field of the Invention
The present invention relates to the technical field of mobile communications and particularly relates to mobile communications systems, base station apparatuses, user apparatuses, and methods.
2. Description of the Related Art
Recently, in this type of technical field, studies on next generation mobile communications systems have been carried out arduously. A representative example of a mobile communications system to succeed the Third Generation (3G) is a Long Term Evolution (LTE) system, which is also called an E-UTRA (Evolved Universal Terrestrial Radio Access) system. In general, the Third Generation system can use a fixed bandwidth of 5 MHz to achieve a transmission rate in the order of up to 2 Mbps in downlink. In the LTE system, a variable bandwidth of 1.4 MHz to 20 MHz may be used to achieve a transmission rate in the order of up to 300 Mbps in downlink and approximately 75 Mbps in uplink. Moreover, as what is to succeed the LTE system, studies are being carried out also for an LTE-Advanced, an IMT-Advanced system, and a Fourth Generation (4G) system (below, the “IMT-Advanced” is referred to as “IMT-A”). A further increase in bandwidth and speed will be achieved in the IMT-A system. Thus, at the present and in the future, it is expected that these multiple systems will co-exist in various places (for the LTE system, see Non-patent document 1, for example.).
FIG. 1 schematically illustrates how the 3G system, the LTE system, and the IMT-A system co-exist in the same region. When multiple new and old systems of different radio access schemes co-exist, it is important to maintain backward compatibility. Maintaining backward compatibility is preferable not only for the user, but also for the operator. When promoting a study of a new system while maintaining backward compatibility, it is particularly important as to how the control signals are transmitted by the old and new systems. However, with respect to how a control signal is transmitted by the new system while maintaining backward compatibility with the LTE system, etc., it appears that studies are not sufficiently being advanced at this time.    Non-patent document 1: 3GPP, TR25.912 (V7.1.0), “Feasibility study for Evolved UTRA and UTRAN, September 2006